The rRNA gene from Schistosoma mansoni has been cloned. The gene is approximately 10K bases in length and is repeated 500-1,000 times. Most copies are identical but divergent copies of the gene have been identified. Different schistosome strains and species can be distinguished by differences in these genes. The surface of the schistosome, represents a prime target for immune attack. Serum from mice vaccinated with highly irradiated cercariae has been utilized to immunoprecipitate surface labelled polypeptides. A dominant species with a molecular weight of approximately 32,000 has thus been identified. Lectin probes have identified glucose and/or mannose, galactose and N-acetylglucosamine as dominat monosacharides on the schistosomula surface. Maturation of the parasite in vivo is concomitant with a decrease in the density of these exposed carbohydrate moieties and an increase in the amount of detectable sialic acid. Monoclonal antibodies have been utilized to probe the nature of the H-2 determinants present on the parasite surface and indicate the presence of intact glycoprotein antigens. The absence of H-2 like genes in the parasite genome suggests that the antigens are acquired from the host and not synthesized by the parasite.